The tires of vehicles represent an elastic link between the wheels and accordingly the vehicle, and the road surface. All the forces for driving the vehicle resulting from propulsion and braking, from changing direction and tracking are transferred frictionally via the tire contact area between the tire and the road surface. The force occurring during this transfer depends primarily on conventional parameters of the tires and the vehicle, however, this force is also determined by the wheel load effective at the moment and adhesive friction value μ, which is subject to abrupt change as a function of weather and the condition of the road surface.
Since the tread element deformation of the tire and the sliding movements between the rubber of the tire and the road surface determine the friction, which makes the transfer of force from the tire to the road possible both in longitudinal and transverse direction, it is necessary to obtain information concerning the transferred force but also concerning the adhesive friction value μ. Therefore, a conventional method is to measure the deformation of individual lug elements of the tire. Starting with the knowledge of the elastic properties of the tire or of the lug elements, it is thus possible to infer the force transferred by the particular lug element. At the same time, however, it is also possible to infer adhesive friction value μ by observing the deformation of a lug element, since during the passage of the particular lug elements of the tire over its wheel contact area on the road surface, local slip events constantly occur, which are a function of the available adhesion potential.
Sensors are conventionally used to obtain information concerning the transferred force and adhesive friction value μ of tires for vehicles. It is thus possible, for example, to measure the deformation of the lug elements of a tire by detecting the movement of a magnet attached there using Hall sensors. According to German Published Patent Application 100 25 502, sensors may be used that may be both capacitive and inductive to measure the deformation of the tread of the tire and accordingly the mechanical stresses in the area of the tire contact area by placing or embedding a plurality of sensors on the tread as well as within the tread elements of the tire. An antenna is assigned to the sensors to supply the measured variables obtained as signals to an analyzing unit. Since multiple sensors are constantly in the reception range of the antenna, it may be necessary to code the sensors in order to obtain specific information concerning the deformation in the tire contact area.
However, it has also been shown that despite the many conventional systems of sensors, even considering their design, an online measurement of the variables of interest with respect to the transferred force and adhesive friction value μ is not possible. Instead, to determine the transferred force and adhesive friction value μ of the tires, it is first necessary to depart from the tire contact surface, so that it is then possible, for example, to determine the force transferred in longitudinal direction using the aligned surface area under the curve of local stress in the lug element as a function of the distance covered. In doing so, it is possible to determine adhesive friction value μ from the specific curve shape, the stress extremes and the slope of the curve in the stress zero crossing in the tire rubber. Nonetheless, however, considerable difficulties result when measuring the transferred force and adhesive friction value μ at low speeds. When the vehicle is standing still, these measurements are not even possible.